A comparison has been made between a calibrated thermopile and an argon ionization chamber as absolute detectors of radiant flux of wavelengths 584 Å and 735 Å. Corrections were applied to the data in order to account for the absorption due to gases in the monochromator, the energy carried away from the thermopile by photoejected electrons, nonuniformity of response across the surface of the thermopile, and uneven illumination of the thermopile by the flux from the monochromator. The two detectors were found to agree within the 3% estimated probable error in the measurements.
L. R. Canfield, R. G. Johnson, K. Codling, and R. P. Madden, "Comparison of an Ionization Chamber and a Thermopile as Absolute Detectors in the Extreme Ultraviolet," Appl. Opt. 6, 1886-1888 (1967)